1. The Field of the Invention
The present invention relates to optical couplers and more specifically to thermally fused couplers for joining segments of fiber optic cable.
2. The Relevant Technology
Fiber optic cable is being used more and more to connect computers to each other and to the Internet. From localized Gigabit Ethernet networks, to wide area networks (WANs) spanning the entire planet, fiber optic cable has become the fastest, easiest way to transmit large amounts of data over either long or short distances. Some fiber optic applications require couplers to route the incoming fiber optic signals to their various destinations.
Fiber optic couplers allow for crossover between the signals on one fiber optic cable and another fiber optic cable. This crossover can also be tuned to allow for certain wavelengths to either cross over or be blocked. However, whether a particular network uses a coupler with a single fiber optic strand or a coupler with many strands, the temperature of the fiber optic cable affects the wavelength propagation properties of the transmitted signal.
An example of a fiber that can be used in a fiber optic coupler is shown in FIG. 1. Two fiber optic lines 10 and 12 are placed side by side. Line 10 has a central section 14 and line 12 has a central section 16. The cladding of central sections 14, 16 is removed and the cores of optical lines 10, 12 can be joined and heated to produce a fused section 18 which acts a fiber optic coupler. Once a cladding layer is applied to fused section 18, a fiber optic coupler 20 is created, as shown in FIG. 2.
In standard couplers, the fused section can then be enclosed in a jacket or ferrule to protect the fused section from untoward bending, which would adversely affect the optical signals traveling through coupler 20. Fiber optic couplers of this type are well known in the art. For a specific method of producing such a fiber optic coupler, see, for example, U.S. Pat. No. 6,431,767.
There are a large number of fiber optic couplers, such as the one illustrated above, available on the market, having various transmission properties that may be desirable for one reason or another. However, all of the couplers currently available suffer, to some extent, from the problem of wavelength shift when the coupler becomes heated.
Most materials have a positive coefficient of thermal expansion, meaning that they expand when they are heated and contract when they cool. Standard fiber optic lines are made from silicon, which has a positive coefficient of thermal expansion. In order to minimize the wavelength shift due to the thermal expansion of the fiber optic cable, conventional systems typically have used a jacket or ferrule over the coupled portion of the fiber optic cable that has approximately the same coefficient of thermal expansion as the fiber optic cable itself. Using a ferrule having approximately the same coefficient of thermal expansion does help alleviate the problem, but it is an imperfect solution. The fiber optic cable still changes shape based on the current temperature. This change in shape still adversely affects the transmission properties of the cable.